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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend> Rear end gear ratio change = spedo. cable gear change. help? turbo 350
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Topic Review (Newest First)
07-18-2007 11:32 AM
Cape Cod Bob Thanx F&J. I will do that right now. My chevy speedometer is also a rotating dial as your Desoto. Thanx again.
07-18-2007 10:54 AM
What is the best way to determine gear ratio?
Of what? the rear axle? If so, jack up one rear wheel on a non-posi car, put a mark on the tire and driveshaft. Then rotate the tire 2 exact turns whil counting how many turns on the driveshaft. If the shaft turns 3 & 3/4 turns, it's a 3.70 ish; if 2 & 3/4 it's around a 2.70 ish.

If it's a posi, jack up both wheels but only turn the tire one turn.

The old speedo should work. I ran a 30 DeSoto with a GM 4 speed trans. The fitting on the old speedo was the same as a more modern car. My old speedo was not a needle, rather a rotating dial.
07-18-2007 09:43 AM
Cape Cod Bob
Gear ratio?

What is the best way to determine gear ratio?
Also, there is no mention of which speedometer one might be using at the web sites posted. I would think that would be important in determining the tranny gears unless all speedometer inards are the same.
I am interested in using the stock speedometer in my 31 chevy with my 350/350 set up. I realize that it only registers on the dial to 85mph but I assume it is some sort of drive that will allow it to register over that without breaking anything.
Of course I NEVER intend on going that fast in my hotrod!
07-18-2007 08:40 AM If you can't find an off-the-shelf replacement gear that will do the trick, go down to your local speedometer/gauge shop (most medium sized towns have one), and have them install a calibration gear box. It is a small custom precision ratio gear reducer that goes in-line with your speedo cable and can be geared for perfect calibration. As a bonus they will supply you with a laminated speedo calibration certification card to show to your friendly Highway Patrol officer when the need arises. I haven't done it in many years but should still be less than $100?
07-18-2007 06:08 AM

This photo shows how everything works ( but the transmission is actually a 700R4 )

This has the gears ...

And here is a on line site with the information on how to figure out the correct gears to get ... ( speedometer recalibration calculator )
07-18-2007 05:34 AM
F&J Yes the driven gears are color coded for the factory workers; They also have the tooth count number marked on them.

A GM dealer, even trucks, may have the old printed parts catalog which gives charts for them. But the older books have tire size listed by type rather than diameters. Like h78-15 etc.

There must still be that info on GM computers at the dealer.

Sometimes a given tire size and ratio will end up to a gear that was never made. In that case, GM used a "speedometer adapter" mini gearbox that fits on the trans. Search ebay for those. The sellers will size it for you.

Also, check your present reading on the highway mile markers. Sometimes the odometer can be Ok but the actual speed reading is wrong. Use a watch to check the actual speed or borrow a GPS that can give speed. Then calculate the error in %, and look on ebay for the matching error correction adapter.
07-18-2007 02:22 AM
sugar ray
Rear end gear ratio change = spedo. cable gear change. help? turbo 350

hey guys and gals!

Just wondering if anyone here has changed a spedo. cable gear before? I changed the gear ratio in my 79 TA. i went from 2.73 to 3.73 gears. waaaaaaaaaaay more bottom end pop. what a world of difference.


since the change, my speedometer is waaaaaaaaaay off. Incorrect reads big time. I realize the solution is to change the plastic gear that is attatched to the spedo cable and resides in the tail shaft of the tranny. My Haynes manual for this car actualy shows 2 plastic gears for the turbo 350. one gear is attatched to where the spedo cable goes into the tail shaft and the other little gear looks like its located above the spedo gear.

Onto to my questions:

Is this little gear easy to change? Is it as simple as disconnecting the drive shaft, popping open the tail shaft cover and installing the proper plastic gear?
or is it much more involved then that? does just the 1 plastic gear have to be chnaged? or do both? lastly, how do i know which gear to order? I realize that they are color coded (red,brown, etc...) which one is color coded for 3.73? or will my dealer know that answer?

looking forward to any input

thanks gang

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